German Bayona¹, William A. Thomas², Stanley C. Finney³, John E. Repetski⁴

(1) Department of Geological Sciences, University of Kentucky, Lexington, KY
(2) Department of Geological Sciences, University of Kentucky, Lexington
(3) Department of Geological Sciences, California State University, Long Beach, CA
(4) U.S. Geological Survey, Reston, VA

ABSTRACT: Flexural Uplifting and Forebulge Migration as Reflection of Collisional Tectonism: An Example from the Alabama Promontory, Southeastern USA

Changes in accomodation space and lithofacies assemblages in response to forebulge migration are used to constrain the history of Taconic orogeny deformation in the southernmost Appalachians. Analysis of Middle and Upper Ordovician strata of the Alabama Appalachians, restored to their respective palinspastic positions and constrained by conodonts, graptolites, and K-bentonites, define two episodes of deformation. The first episode is recorded by a diachronous up-section enrichment of skeletal fragments in upper Middle Ordovician carbonate strata followed by deposition of graptolitic shale. The carbonate-clastic contact is sharp near the margin, but it is gradational toward the craton where skeletal-rich beds are thicker and better sorted. Scarce, thin bentonite beds and soft-sediment deformation attended the first episode. In early Late Ordovician time, an upward-shallowing trend of siliciclastic deposition in the foreland and an upward-deepening trend of carbonate deposition toward the craton record the second episode. K-bentonite beds are thicker and less stratigraphically dispersed toward the craton.

Depositional trends and lithofacies assemblages of Middle and Upper Ordovician strata of Alabama document two forebulges. The first forebulge is recognized from diachronous shallowing and abrupt deepening of depositional conditions in the late Middle Ordovician. Sorting of skeletal grains and mud winnowing were enhanced as the forebulge migrated across the shallower platform. The second forebulge is identified from inversion of bathymetry in the Late Ordovician. Dispersal of clastic detritus along the forebulge favored carbonate deposition toward the craton. Late Ordovician depositional trends are masked somewhat by a craton-wide rise of sea level and reactivation of basement faults.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado